Biomass refers to biological material derived from living or deceased organisms and includes lignocellulosic materials (e.g., wood), aquatic materials (e.g., algae, aquatic plants, and seaweed), and animal by-products and wastes (e.g., offal, fats, and sewage sludge). In the conventional pyrolysis of biomass, typically fast pyrolysis carried out in an inert atmosphere, a dense, acidic, reactive liquid bio-oil product is obtained, which contains water, oils, and char formed during the process. Much of the oxygen present in biomass ultimately resides in the bio-oil, thereby increasing its chemical reactivity.
Characteristic total acid numbers (TAN) of conventional bio-oil are in the range of 100-200, rendering it highly corrosive. Moreover, this product tends to undergo polymerization, is generally incompatible with petroleum hydrocarbons due to water miscibility and very high oxygen content (on the order of about 40% by weight), and has a low heating value. The unstable bio-oils of conventional pyrolysis tend to thicken over time and can also react to a point where hydrophilic and hydrophobic phases form. As a result, transportation and utilization of this product are problematic. Also, it is difficult to upgrade this product to a liquid hydrocarbon fuel, due to the retrograde reactions that typically occur in conventional pyrolysis processes, including fast pyrolysis. Dilution with methanol or other alcohols has been shown to reduce the activity and viscosity of the formed bio-oils, but this approach is not considered practical or economically viable, due to the large amounts of unrecoverable alcohol that are required to stabilize pyrolysis liquids.
The removal of char, generated by conventional pyrolysis, from the liquid pyrolysis product while it is still in the vapor phase presents an additional technical challenge. Significant amounts of oxygen and free radicals in the pyrolysis vapors remain highly reactive and form a pitch-like material upon contact with char particles on the surface of a filter or other solid separator. Consequently, devices used to separate char from the hot pyrolysis vapors can become quickly plugged, due to the reactions of char and pyrolysis vapor constituents that occur on and within the layer of char on the surfaces of such devices, as well as within the pores of porous filter elements. Finally, it is noted that the upgrading of pyrolysis oils, using conventional hydroconversion processes, consumes large quantities of H2 and extreme process conditions, including high hydrogen pressures needed to meet product quality requirements, make such processes uneconomical. The reactions are inherently out of balance in that, due to the high pressures required, too much water is created while too much H2 is consumed. In addition, conventional hydroconversion reactors can rapidly develop high pressure differentials, due to reactive coke precursors present in the pyrolysis oils or from coke produced as a result of catalysis.
More recently, the use of hydrogen in biomass pyrolysis (i.e., hydropyrolysis) has been disclosed. For example, hydropyrolysis processes taught in U.S. Pat. No. 8,492,600 have been found to overcome a number of the drawbacks of conventional fast pyrolysis processes, including those described above, and have led to a number of other processing advantages.
Various feedstocks may comprise biomass, but in many cases also contain significant amounts of other materials that present technical challenges to converting such feedstocks to higher value liquid products, including hydrocarbon-containing products useful for transportation fuels. Municipal solid waste (MSW), for example, can present an essentially no-cost source of biomass (e.g., waste wood, yard waste, and agricultural waste) and biomass-derived materials (e.g., paper, cardboard, medium density fiberboard (MDF), and particleboard). MSW may, however, also contain significant amounts of non-biological materials, e.g., plastic, glass, waste metal, etc., which are derived from petroleum or minerals. Ash, which refers to the non-combustible solid powder residue (generally containing metals and/or metal oxides) left behind following combustion, is present to some extent in biomass as a non-biological material. However, the presence of other non-biological materials in MSW can lead to a higher overall ash content, relative to that of the biomass portion of the MSW alone. Non-biological materials in MSW complicate upgrading processes, including those that involve pyrolysis in the presence of hydrogen (hydropyrolysis), and optionally other processing steps leading to the production of higher value liquids such as transportation fuel fractions.
The presence of non-biological materials in MSW can additionally alter the overall atomic ratios present in the feedstock, compared to the atomic ratios of the biomass portion of the MSW alone. For example, the ratio of oxygen to carbon in the feedstock may be reduced, thereby affecting the reactions occurring during pyrolysis, and/or subsequent processing. This, in turn, can adversely impact the quality of the products obtained and/or otherwise significantly increase the costs associated with obtaining products of given quality and yield. The bulk mechanical properties of MSW feedstock, as it undergoes heating to carry out desired conversion steps (e.g., fluidized bed hydropyrolysis), may also differ significantly from the mechanical properties of purely biomass-containing feedstocks, such as lignocellulosic materials. The differing process conditions and/or additional process steps, as needed to compensate for these characteristics of MSW, relative to conventional feeds, can vary greatly. This is particularly the case in processes for converting MSW to liquid products that include hydropyrolysis and optionally other upgrading steps.
More recently, the potential for using algae and lemna (sea weeds) as a source of biomass for producing higher value liquids has gained attention. Algae grown in salt water may, however, contain large amounts of sodium and chlorine relative to other sources of biomass (e.g., wood). Sodium is a potential catalyst poison that may be detrimental to the activity of catalysts used in biomass conversion processes, such as catalytic hydropyrolysis and/or catalytic hydroconversion (e.g., hydrodeoxygenation). The existence of chlorine in a hydrogen environment, and particularly in the presence of a catalyst with hydrogenation activity, can lead to the formation of hydrogen chloride gas (HCl). If water is also present, the condensation of aqueous HCl (i.e., hydrochloric acid) on metallic surfaces can lead to the rapid corrosion of metallic process vessels and even catastrophic failure. Higher grades of metallurgy (e.g., Hastelloy®) over stainless steel can offer some protection, but only at significantly increased capital expense. With respect to MSW feedstocks, the presence of relatively high amounts of other heteroatoms, such as sulfur and nitrogen, under hydrogenation conditions leads to the formation of H2S and NH3 that likewise pose concerns in terms of their corrosivity and potential detrimental health effects.
Therefore, while MSW and algae provide attractive, low-cost (or no-cost) feedstocks for producing green energy with little or no associated greenhouse gas (GHG) emissions, they also present the technical challenges identified above. There is consequently a need in the art for processes that address some or all of these challenges, and particularly processes involving the hydropyrolysis of feedstocks containing non-biological materials. The ability to transform readily available raw materials, including materials that are otherwise regarded as waste products, to provide higher value liquids (e.g., hydrocarbon-containing liquids), would represent a major breakthrough in realizing cost-competitiveness with conventional petroleum refining processes.